Method for the Destruction of Organic Contaminants through Smoldering Combustion

ABSTRACT

A method for remediating contaminated soil and groundwater includes selecting a treatment material and creating a smolderable mixture of a contaminant, the treatment material, and soil.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of application Ser. No.16/394,245, filed on Apr. 25, 2019, which claims the benefit of U.S.Provisional Patent Application 62/663,355 filed Apr. 27, 2018, thedisclosure of which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to methods for the destruction of organiccontaminants through smoldering combustion, including the in situ and exsitu mixing of a porous matrix and organic treatment materials tofacilitate the smoldering combustion process.

BACKGROUND ART

Smoldering combustion, when applied for the remediation of contaminatedsoils, is known commercially as the Self-sustaining Treatment for ActiveRemediation (STAR) technology and is the subject of U.S. Pat. No.8,132,987.

Smoldering combustion requires a short duration energy input, and theaddition of an oxidant (e.g., oxygen, air, etc.), to initiate andsustain the smoldering combustion reaction. Smoldering combustion isonly possible in the presence of a fuel source and a porous matrix. Acommon example of a smoldering combustion reaction is that of a burningcharcoal briquette where the charcoal is both the fuel and the porousmatrix. For the STAR process, however, the fuel is the organiccontaminant and the porous matrix is the subterranean volume of soilundergoing remediation.

Smoldering combustion of the organic contaminant may be self-sustainingin that it may only be necessary to supply sufficient energy to ignitethe material; once ignited, combustion of the material may proceed aslong as there is sufficient fuel (the combustible material) and oxygenfor combustion to take place. This is in contrast with, for example,known thermal remediation processes, which require continuous energyinput.

If sufficient fuel is not present (i.e., the contaminant concentrationor saturation in soil is too low to support a self-sustaining combustionreaction), a surrogate fuel may be added to the fuel/porous matrixmixture as described for the ex situ treatment of soils and liquidorganic wastes patent “Method for Volumetric Reduction of and OrganicLiquid” (Australian Patent Number 2012249643). This process is knowncommercially as ex situ Self-sustaining Treatment for Active Remediation(STARx)

There are a variety of organic contaminants that are resistant tocomplete destruction by chemical, biological, or other means. Examplesof recalcitrant compounds include per- and polyfluoroalkyl substances(PFAS), dioxins, and polychlorinated biphenyls (PCBs). These types ofrecalcitrant compounds, however, may be amenable to destruction viasmoldering combustion or decomposition at elevated temperatures.

In the field of groundwater and soil remediation, there are many in situtechniques available to capture and treat recalcitrant organiccontaminants in groundwater and soil. One technique is the use ofPermeable Reactive Barriers (PRBs) that destroy, detoxify, collect orimmobilize dissolved contaminants in groundwater as it flows through thePRB. The treatment materials used in PRBs may: (1) chemically react withdissolved contaminants (e.g., zero-valent metals), (2) adsorb dissolvedcontaminants (e.g., charcoal, activated carbon, resins, polymers);and/or (3) enhance the biological degradation processes of the dissolvedcontaminants (e.g., vegetable oil acting as an electron donor forreductive processes). A PRB can be 100% of the selected treatmentmaterial or mixed with soils or other permeable materials forconstruction or performance purposes. PRBs can also be composed ofmixtures of treatment materials.

Treatment materials can also be added directly to the source zone ofcontaminants so that treatment can begin immediately upon application ofthe treatment materials rather than rely on the transport ofcontaminants to the location of the PRB. The treatment materials used insource zones can be the same or similar materials to those used in PRBsto cause chemical reaction, adsorption or biodegradation processes, butmay also include clays or cements.

Some common treatment materials are organic and therefore combustible,such as vegetable oil or granular activated carbon. These organictreatment materials can be smoldered in a self-sustaining reaction andare common fuel surrogates for the ex situ destruction of organiccontaminants via STARx. When these organic treatment materials are usedin a PRB, low concentration or recalcitrant contaminants, such as PFAScompounds dissolved in groundwater and accumulating in the PRB, can bedestroyed (i.e., thermally decomposed) through the in situ or ex situsmoldering combustion of the mixture of the contaminant, organictreatment material, and/or soils or other permeable constructionmaterial. Likewise, when these organic treatment materials are mixedinto contaminant source zones, destruction (i.e., thermal decomposition)of the contaminant can take place through the smoldering combustion ofthe mixture of contaminant, organic treatment material, and/or soils orother permeable construction material.

SUMMARY OF THE EMBODIMENTS

In accordance with one embodiment of the invention, a method forremediating contaminated soil and groundwater includes selecting anorganic treatment material and creating a smolderable mixture of acontaminant, the organic treatment material, and soil. The methods alsoheats a portion of the smolderable mixture, forces oxidant through thesmolderable mixture to initiate a self-sustaining smoldering combustionof the smolderable mixture to destroy or remove the contaminant, andterminates the source of heat applied to the smolderable mixture. Themethod may propagate the combustion away from the point of ignition ofthe combustion.

Contaminants that can be treated by the method include but are notlimited to perfluoroalkyl substances, polyfluoroalkyl substances,dioxins, polychlorinated biphenyl compounds, pesticides, herbicides,volatile organic compounds (VOCs), and semi-volatile organic compounds(SVOCs), and combinations of these and other contaminants.

The organic treatment material may be vegetable oil, hydrocarbons, tar,activated carbon, coal, charcoal, polymers, surfactants, woodchips andcombinations thereof The organic treatment material may be admixed belowthe ground to form a smolderable mixture using a variety of methods suchas trenching, large diameter auger, excavation, caisson, injection,jetting, fracking, vibrating beam, tremmie, soil mixing and combinationsthereof. The organic treatment material may be a liquid, a slurry, or asolid. The smolderable mixture thus formed may be smoldered belowground, or may be removed and smoldered above ground. The organictreatment material may also be mixed above ground with soil containingthe contaminant to form a smolderable mixture for smoldering aboveground.

In some embodiments, the smolderable mixture may be part of a permeablereactive barrier that intersects the groundwater that contains thecontaminants. The smolderable mixture may absorb and concentrate thecontaminants, allowing for the removal of the contaminants fromgroundwater and thereafter the destruction or removal of thecontaminants by smoldering. The soil containing the contaminants may bemixed below ground with the organic treatment material to create asmolderable mixture. The smolderable mixture may be combusted in placeor removed and combusted above the ground. Alternatively, the soiltrapped in the permeable reactive barrier containing the concentratedcontaminant may be removed, mixed with organic treatment material aboveground, and smoldered above ground.

As a means to initiate smoldering at lower temperatures, and as a meansto propagate smoldering, oxidant may be forced through the smolderablemixture by injecting air into the smolderable mixture through one ormore injection ports and/or by creating a vacuum to suck oxidant throughthe smolderable mixture. The oxidant may be forced through thesmolderable mixture at a linear velocity of between 0.0001 and 100centimetres per second.

A self-sustaining smoldering combustion may be achieved by applying heatto the smolderable mixture from at least one internal conductive heatingsource in direct contact with the smolderable mixture, or at least oneconvective heating source coupled to the smolderable mixture. Theconvective heating source coupled to the smolderable mixture may beexternal to the mixture or located within the smolderable mixture. Aself-sustaining smoldering combustion may be also be achieved byapplying radiative heat to the smolderable mixture. Smolderingcombustion may be performed at a temperature within a range between 200and 2000 degrees Celsius.

In an embodiment of the invention there is provided a method foremplacing the organic treatment material in a manner that forms asmolderable mixture below ground that can trap (e.g., absorbs) dissolvedcontaminants and/or that can encapsulate the volume of soil thatcontains contaminants.

In other embodiments, the smolderable mixture makes up a PRB thatintersects the groundwater containing the dissolved contaminants.

In other embodiments, the volume of soil containing the contaminants ismixed with organic treatment material to create a smolderable mixture.This mixing can take place below ground or above ground, and smolderingcan occur below ground or above ground.

In other embodiments, the smolderable mixture absorbs and concentratesthe contaminants allowing their removal from water and thereafter theirdestruction or removal by smoldering.

In other embodiments, the absorption and concentration of thecontaminants facilitates the smoldering combustion process of thesmoldering mixture.

In other embodiments, the combustion of the smolderable mixture createstemperatures that destroy or remove contaminants within the smolderablemixture.

In other embodiments, after combustion, additional organic treatmentmaterial can be added to the PRB or the volume of soil containing thecontaminants for additional treatment.

In other embodiments the organic treatment material can be added to aPRB or source below the ground using a variety of methods, for example,trenching, large diameter auger, excavation, caisson, injection,jetting, fracking, vibrating beam, tremmie, and soil mixing. Thesmolderable mixture can be created above ground and emplaced belowground by the variety of methods or can be emplaced and mixed directlybelow ground.

In other embodiments, the smolderable mixture can be combusted in place(i.e., in situ).

In other embodiments, the smolderable mixture can be removed andsmoldered above ground (i.e., ex situ). A smolderable mixture can alsobe formed by removing soil containing contaminants, mixing this soilabove ground with organic treatment material, to create a smolderablemixture for smoldering above ground.

In general terms, in each of the above described embodiments, it isdesired to create a smolderable mixture through the addition of anorganic treatment material and promote/maintain self-sustainedsmoldering combustion of the smolderable mixture as a method to destroyor remove the contaminant(s) in the PRB or in the volume of soilcontaining the contaminants.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of embodiments will be more readily understood byreference to the following detailed description, taken with reference tothe accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a prior art PRB thatcontains a smolderable mixture of contaminants, organic treatmentmaterial and porous matrix material.

FIG. 2 is a schematic cross-sectional view of a volume of soilcontaining contaminants, organic treatment material and porous matrixmaterial.

FIG. 3 is a schematic cross-sectional view of PRB being installed thatis composed of a smolderable mixture by various trenching and excavationtechniques.

FIG. 4 is a schematic cross-sectional view of PRB being installed thatis composed of a smolderable mixture by caisson techniques.

FIG. 5 is a schematic cross-sectional view of a PRB being installed thatis composed of a smolderable mixture by jetting or fracking techniquesor use of mixing tools.

FIG. 6 is a schematic cross-sectional view of a soil mixing techniquebeing used to install a smoldering mixture into a volume of soilscontaining contaminants.

FIG. 7 is an enlarged schematic view of a smolderable mixture accordingto embodiments of the invention.

FIG. 8 is a schematic cross-section of a PRB containing a smolderablemixture undergoing a treatment by smoldering combustion comprising anoxidant source and heating elements.

FIG. 9 is a cross-sectional schematic of a contaminated volume of soilcontaining a smolderable mixture undergoing treatment by smolderingcombustion comprising an oxidant source and heating elements

FIG. 10 is a flow diagram illustrating particular steps according to theembodiments of the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Embodiments described herein rely on the principles of self-sustainedsmoldering combustion for the remediation of soil and groundwatercontaminants which provides benefits over currently availablegroundwater and soil remediation techniques for contaminants that areconsidered recalcitrant but can typically be destroyed at temperaturesabove 250° C. In one embodiment, a smolderable mixture is formed frommixing an organic treatment material (e.g., activated carbon, charcoal,vegetable oil, polymers, surfactants alone or in combination), that initself may or may not be porous, a porous matrix (e.g., soil, sand), anda contaminant (either dissolved in groundwater, sorbed to soil, orpresent as a separate phase), which is combusted via self-sustainingsmoldering combustion to destroy the contaminant. In some embodiments,the organic treatment material itself is porous, and a separate porousmatrix is not necessary. In other embodiments, the organic treatmentmaterial is mixed with a porous matrix such as sand or soil.

Embodiments of the present invention are based on using smolderingcombustion to destroy or remove dissolved, sorbed, or separate phasecontaminants below ground. Smoldering refers to combustion of a materialat the surface of the solid or liquid material itself. For example, whena combustible material (e.g., tobacco) is compacted to form a poroussolid (e.g., a cigarette) and is ignited, the oxidant (e.g., oxygen)diffuses into the surface of the material and the combustion proceeds atthe surface of the tobacco leaf fragment. Smoldering is referred to as aheterogeneous combustion reaction because the oxidant (gas) and the fuel(liquid or solid) are distinct phases. This is in contrast to flamingcombustion which is a homogeneous reaction occurring in a single (gas)phase.

The smoldering combustion process results in the generation of energy,water, and vaporous emissions, primarily carbon dioxide, carbonmonoxide, and to a lesser extent volatile organic compounds and othercompounds depending on the composition of the contaminants and solidmaterial.

In embodiments of the present invention, the smolderable mixture servesas a scaffold to both entrap the contaminants that are to be treated andan environment that facilitates smoldering combustion. Smolderingcombustion is maintained through the efficient recycling of energywithin the system. First, the organic treatment material within thesmolderable mixture, and/or the organic contaminants that areconcentrated in the smolderable mixture is combusted, giving off heatenergy which is retained by the porous matrix. Second, the retained heatenergy is returned to the system from the porous matrix to pre-heat anyother organic material within the smolderable mixture farther removedfrom the point in space where the combustion process was initiated.Thus, following a short duration energy input to initiate the process,smoldering combustion is self-sustaining (i.e., it uses the energy ofthe combusting organic materials—contaminants and/or organic treatmentmaterials—along with a supply of oxidant, to maintain the reaction) andis capable of propagating away from the point of ignition through thesmolderable mixture. Smoldering is the only type of combustion reactionthat can propagate through an organic material/porous matrix mixture(i.e., flames are not capable of propagating through such a system). Ina self-sustaining process, the heating source is terminated followingthe initiation of smoldering combustion.

The self-sustaining smoldering combustion process will also generatesufficient temperatures to destroy or remove organic contaminants thatare within the smolderable mixture if the following conditions are met:(1) the organic material (contaminants and/or organic treatmentmaterials) contains sufficient inherent energy to sustain a smolderingcombustion process (i.e., it is a combustible material); (2) it is aporous matrix itself or is mixed with a porous matrix to enable thesmoldering process; (3) a heat source is provided to initiate theprocess; and (4) at least one oxidant (e.g., oxygen, air) initiates andmaintains the process.

The self-sustaining smoldering combustion treatment method applies toeither solid or liquid organic materials and can be conducted insynthetic or natural porous medium or granular solid matrices.

Definitions. As used in this description and the accompanying claims,the following terms shall have the meanings indicated, unless thecontext otherwise requires.

As defined herein, the terms per- and polyfluoroalkyl substances areaccording to the definitions provided by R. C. Buck, et al.,Perfluoroalkyl and Polyfluoroalkyl Substances in the Environment:Terminology, Classification, and Origins, 2011, Integrated EnvironmentalAssessment and Management, Volume 7, Number 4, pg. 515.

The term “perfluoroalkyl substance” refers to an aliphatic substance forwhich all of the H atoms attached to C atoms in the nonfluorinatedsubstance from which they are notationally derived have been replaced byF atoms, except those H atoms whose substitution would modify the natureof any functional groups present.

The term “polyfluoroalkyl substance” refers to an aliphatic substancefor which all H atoms attached to at least one (but not all) C atomshave been replaced by F atoms, in such a manner that they contain theperfluoroalkyl moiety C_(n)F_(2n+1)—.

The term “PFAS” refers, collectively, to perfluoroalkyl substances andpolyfluoroalkyl substances.

The term “porous matrix” refers to a synthetic or natural solid materialhaving pores (open spaces) and wherein the solid material may be asingle piece having pores or a collection of granular solids havingpores there between. Examples of materials suitable of comprising theporous matrices of embodiments of the present invention include sand,gravel, glass beads, wood chips, zeolite, activated carbon, charcoal,soil, crushed stone, ceramic chips or beads, and combinations thereof.The porous matrix may be organic and therefore combustible or inorganicand not combustible.

The term “smoldering combustion” refers to the act or process of burningwithout flame; a rapid oxidation accompanied by heat and light but notflame. In smoldering combustion, the combustion occurs on the surface ofthe fuel (i.e., not in the gas phase above the fuel as with a flame), inthis case, the organic material.

The term “organic material” refers either a liquid or a solid containingorganic carbon compounds or combustible compounds.

The term “treatment material” refers to either a liquid or a solidcompound that is emplaced in the subsurface to destroy, detoxify,collect or to provide a porous matrix to immobilize contaminants.Examples of treatment materials include: vegetable oil, hydrocarbons,tar, activated carbon, coal, charcoal, polymers, surfactants, sand,gravel, glass beads, wood chips, zeolite, soil, silt, fill, crushedstone, ceramic chips or beads, and combinations thereof. In someembodiments, the treatment materials may contain carbon or organiccompounds. In some embodiments, the treatment material may be or mayinclude combustible compounds that can be used as a fuel source forsmoldering combustion to destroy or remove a subsurface contaminant. Insome embodiments, the contaminant itself provides the fuel source forsmoldering combustion. Treatment materials are not the contaminant beingremoved by the method.

The term “organic treatment material” refers to treatment materialscomprising organic compounds, for example, vegetable oil, hydrocarbons,tar, activated carbon, coal, charcoal, polymers, surfactants, woodchipsand combinations thereof. Some organic treatment materials provide bothfuel and a porous matrix.

“Self-sustaining” refers to reaction conditions wherein smolderingcombustion propagates through the organic material without theapplication of external energy; that is, when the already smolderingorganic material produces sufficient heat to elevate the temperature inthe adjacent material to its combustion point. Conditions may beself-sustaining even if initially the application of heat is required toinitiate smoldering combustion.

The term “smolderable mixture” refers to any mixture of porous matrix,organic material, or conglomeration or aggregation of a porous matrixmaterial that supports smoldering combustion.

The term “ignition” refers to the process of initiating smolderingcombustion.

The term “conductive heating” refers to the transfer of thermal energyby direct physical contact.

The term “convective heating” refers to the transfer of thermal energyby the movement of fluids.

The term “radiative heating” refers to the transfer of thermal energy byelectromagnetic radiation.

The term “PRB” means a permeable reactive barrier that allowsgroundwater to flow through it and can destroy, detoxify, collect orimmobilize contaminants. Accordingly, the PRB may contain an organictreatment material. In some embodiments, the PRB may also perform thefunction of being used as a fuel source for smoldering combustion todestroy or remove a subsurface contaminant.

The porous matrix may be the organic material.

The smolderable mixture emplacement may be achieved manually, viabackhoe or excavator, jetting, fracking, trenching, soil mixing or othermethods.

Many organic treatment materials may be used as the fuel source forsmoldering combustion by the methods disclosed herein. Examples oforganic treatment materials for which the methods are particularlyeffective include hydrocarbon mixtures such as coal, activated carbon inall forms, shredded tires, wood, char and vegetable oils.

In embodiments of the invention, the following porous matrix materialshave been found to form suitable admixtures with organic materials:sand, gravel, ceramic beads, porous metals, porous ceramics, coal,charcoal, activated carbon, and glass beads. These materials, if sizedcorrectly, have a high surface area to volume ratio such that asufficient amount of heat generated during the combustion process istransferred to and stored in the matrix material, so as to make the heatstored in the matrix material available to assist in further combustionof the organic material. The matrix material has further characteristicsof sufficient pore space to receive organic material admixed therewith,and surface, shape, and sorting characteristics that are amenable to airflow through the pore spaces.

Ignition of smoldering combustion requires both a heating source toinitiate combustion and a source of oxidant to initiate and maintaincombustion.

FIG. 1 illustrates a PRB 11 containing a suitable mixture 12 locatedbelow ground 13 and at and below the water table 14. The PRB can beextended into the vadose zone 15 and be composed of contaminants,organic treatment material, and/or soils or other permeable constructionmaterial.

FIG. 2 illustrates that the suitable mixture can be emplaced in thevadose zone 21, below the water table 22 or from the surface into thewater table 23.

FIG. 3 illustrates a method using excavation or trenching methods tomake a PRB 31 composed of organic treatment material, and/or soils orother permeable construction material 32. The PRB may contain excavatedsoils 33. The PRB can be emplaced with equipment such as backhoe 34 andtremmie tube 35 methods.

FIG. 4 illustrates the use of caisson methods to emplace a mixture oforganic treatment material and/or soils or other permeable constructionmaterial. A caisson 41 is advanced to remove soil 42. The excavated soilis mixed with organic treatment material and/or other permeableconstruction materials to create a smolderable mixture 43 or apreviously prepared smolderable mixture 43 is then used to fill thespace in the caisson. The caissons can be discrete or overlapped tocreate a continuous zone comprised of the suitable mixture.

FIG. 5 illustrates the use of soil mixing 54, jetting methods 55, orinjection methods 56 to add a smolderable mixture 53 to create a PRB.

FIG. 6 illustrates the use of soil mixing 64, jetting methods 65 orinjection methods 66 to mix organic treatment material 63, and/or soilsor other permeable construction material into a volume of soilcontaining the contaminants 62. FIG. 6 also shows a mixture 61 havingbeen mixed into the contaminated materials (wherein the mixture may havebeen formed from the methods shown in FIG. 5).

FIG. 7 illustrates a suitable mixture composed of organic treatmentmaterials 71 within a porous matrix 72, and the pore space of thesuitable mixture may contain air or water, or organic contaminants inany proportion.

FIG. 8 illustrates the application of smoldering combustion to treatcontaminants in a PRB. Oxidant is supplied to the PRB from an oxidantsource 81 through injection points that may be vertical or horizontal 82located within the PRB. The air injection points may comprise a singleaperture into the PRB or may have multiple points placed within the PRB.Various heating sources (e.g., conductive, convective, inductive, orradiative) may be used either alone or in combination for ignition ofsmoldering combustion. For example, a heating source 83 may be placedin-line with the supplied oxidant to supply heat to the suitablemixture. Heating sources 86 may also be positioned within the PRB.Additionally, an internal heating source 87 may be placed within the airsupply port. The heating element may be an electrically-powered cableheater, electrically-powered cartridge heater, electro-magneticallyactivated heating system, or radiative tube heater in which propane orother external fuel source is internally supplied and combusted. Vaporsand products of the combustion reaction can be collected from the PRBwith a vapor collection system that collects vapors below ground 84 orabove ground 88 and routed for treatment by means of a routing system 85or released to the atmosphere.

FIG. 9 illustrates the application of smoldering combustion to treat avolume of soil containing contaminants, organic treatment material,and/or soils or other permeable construction material. Oxidant issupplied to the PRB from an oxidant source 91 through an injectionpoints that may be vertical or horizontal 92 located within the PRB. Theair injection points may comprise a single aperture into the PRB or mayhave multiple points placed within the PRB. Various heating sources(e.g., conductive, convective, inductive, or radiative) may be usedeither alone or in combination for ignition of smoldering combustion.For example, a heating source 93 may be placed in-line with the suppliedoxidant to supply heat to the suitable mixture. Heating sources may alsobe positioned within the volume of soil containing contaminants 96.Additionally, an internal heating source may be placed within the airsupply port 97. The heating element may be an electrically-powered cableheater, electrically-powered cartridge heater, electro-magneticallyactivated heating system, or radiative tube heater in which propane orother external fuel source is internally supplied and combusted. Vaporsand products of the combustion reaction can be collected from the PRBwith a vapor collection system that collects vapors below ground 94 orabove ground 98 and routed for treatment 95 or released to theatmosphere.

FIG. 10 illustrates the key steps in the invention. As shown in thefigure, an organic treatment material is first selected. This step isfollowed by a mixing step wherein the contaminant is mixed with theorganic treatment material and/or soils or other permeable constructionmaterial. Smoldering combustion is then initiated and maintained. Ifmore treatment is needed, additional organic treatment material can beselected and the steps repeated as per the figure.

The air supply points may be perforated plates, screens, perforatedcarbon-steel, stainless-steel or other material rods, carbon-steel,stainless-steel or other material wells with wire-wrapped or slottedscreens installed within the vessel. The heating elements may beelectrical resistive heaters or radiative heaters installed or placedwithin or adjacent to the air supply ports, installed in or adjacent tothe mixture surrounding the supply ports, or an element heating airpassing through the supply ports and into the mixture.

In particular embodiments, the oxidant is oxygen supplied as a componentof atmospheric air. The reaction is controllable such that terminatingthe supply of oxygen to the reaction front terminates the reaction.Increasing or decreasing the rate of oxygen flux to the reaction frontwill also increase or decrease the rate of combustion and, therefore,the propagation rate of the reaction front, respectively. Also, the airsupply can be enriched with additional oxygen to increase the oxygencontent of the air supplied.

It should be appreciated that combustion can be monitored according tomethods known to those of skill in art to determine the amounts ofoxygen, air or other oxidant required to maintain smoldering combustion.Combustion temperatures are commonly monitored with thermocouples whichcan be placed throughout the volume of material being combusted.

Combustion gases and other compounds produced by the process arecollected at the outlet vapor collection points above or at the surfaceof the PRB or volume of soils containing the contaminants.

The air supply points may be spaced according to the overall dimensionsof the volume of soil containing the contaminants or the PRB so thatoxidant is delivered in sufficient quantity and at a sufficient ratethroughout the volume of soil containing the contaminants or the PRB;thereby facilitating smoldering combustion throughout the volume of soilcontaining contaminants or the PRB.

The embodiments of the invention described above are intended to bemerely exemplary; numerous variations and modifications will be apparentto those skilled in the art. All such variations and modifications areintended to be within the scope of the present invention as defined inany appended claims.

EXAMPLES

The following examples relate specifically to experiments addressing theremediation of PFAS contaminants using smoldering combustion. Theseexperiments were performed with funding from the Strategic EnvironmentalResearch and Development Program (SERDP), Project Number ER18-1593. Thefinal report from this project is incorporated herein by reference(Final Report, SERDP Project number ER18-1593).

The temperatures obtained through smoldering can be significant, andexceed temperatures needed to destroy PFAS. However, unlike solid orliquid fuels, PFAS are not contaminants that can support smolderingcombustion in and of themselves. Example 1 evaluates the ability of twospecific organic treatment materials to act as fuel to support the hightemperature smoldering process (greater than 900° C.) sufficient todestroy PFAS. The two organic treatment materials examined were granularactivated charcoal (GAC) and crumb rubber. Both GAC and crumb rubberwere found to work as organic treatment materials for PFAS destruction.Example 2 assesses PFAS destruction during smoldering combustion usingGAC as an organic treatment material.

Example 1: Testing of Organic Treatment Materials

The purpose of this Example was to determine a fuel (i.e. organictreatment material) and fuel ratio that would achieve temperaturesufficient to destroy PFAS. During these tests, fuel, ratio of fuel tosand, and air flux were varied to understand the relationships betweenthese parameters and the resulting temperatures recorded duringsmoldering tests. For these tests, two fuels were identified aspotential organic treatment materials: granular activated carbon (GAC)and crumb rubber.

Column Test Setup

Laboratory tests used a stainless-steel column that had a diameter of 16cm and a height of 60 cm. The smoldering column was placed in a walk-infume hood on a scale which recorded the mass loss in real-time. A coiledresistive heater (450 W, 120 V, Watlow Ltd.) was located at the base ofthe column and was connected to a single-phase variable power supply(120 V, STACO Energy Products). The air supply was connected at the baseof the column; a layer of coarse and medium sand was placed above theair supply to evenly distribute the airflow throughout the cross-sectionof the column. Thermocouples were inserted horizontally at 3.5 cmincrements vertically up the column and measured temperatures at thecenter of the column. The first and second thermocouples were placedjust below and above the heater. The stainless-steel column was wrappedwith insulation (5 cm thick mineral wool, part number 9364K62,McMaster-Carr, Aurora, Ohio) to minimize heat losses. Gas emissions wereanalyzed for volume fractions of oxygen, carbon monoxide, and carbondioxide using a gas analyzer. During tests, a data logger and personalcomputer recorded the mass loss, thermocouple, and gas analyzer data.Measurements were taken approximately every two seconds.

Preparing the Smoldering Organic Treatment Material and Sand Mixture

The stainless-steel column was packed with approximately 25 cm of coarsesilica sand and GAC or crumb rubber (mesh 10-20, Emterra). The GAC orcrumb rubber and coarse silica sand were mixed at predetermined ratiosfor the test, using a KitchenAid mixer (Professional 600™) to create auniform mixture. Two batches of approximately 4 kg each were required tocreate a 25 cm pack height in the smoldering column. 14 cm of coarsesand was added into the column above the fuel and sand mixture to allowfor cooling.

Smoldering Procedure

The heater was turned on to begin pre-heating. Once the secondthermocouple reached 260° C., air was introduced into the bottom of thecolumn using a mass flow controller (FMA5400/5500 Series, Omega Ltd.).The heater was turned off when the third thermocouple peaked.Self-sustaining smoldering was then shown by the consecutive consistentpeak temperatures exhibited by the thermocouples in the fuel and sandmixture. The self-sustaining smoldering continued until the frontreached the clean sand layer; at this time, the temperatures began todecrease.

Results and Discussion

Table 1 presents a summary of eight tests conducted with GAC and crumbrubber. Although crumb rubber (test 1-2) supported self-sustainedsmoldering, it produced emissions with aerosol droplets (condensates)that made it not a suitable candidate for further testing.

TABLE 1 Test Summary Average Ratio Air Average Front Smoldering (g fuel/Flux Moisture Peak Temp. Velocity Temperature Self- Test No kg sand)Fuel (cm/s) Content (%) (° C.) (cm/min) Trend Sustaining 1-1 60 GAC 5 01253 0.64 Steady Yes 1-2 60 Crumb 5 0 720 0.64 Increasing Yes Rubber 1-340 GAC 5 0 990 0.66 Increasing Yes 1-4 20 GAC 5 0 690 0.49 Steady Yes1-5 40 GAC 2.5 0 1003 0.47 Steady Yes 1-6 40 GAC 7.5 0 1044 0.7 SteadyYes 1-7 60 GAC 2.5 0 1153 0.39 Steady Yes 1-8 20 GAC 2.5 0 661 0.33Steady Yes

Analysis of these data demonstrate that the peak temperature iscontrolled in an approximately linear relationship by the GACconcentration. There is minimal influence on peak temperature by airflux. Conversely, the front velocity is controlled, again in anapproximately linear fashion by air flux.

These results support the conclusion that there is an energy balancethat is dominated by the rate of oxidation and heat release and heatloss. As the fuel content increases of a given fuel, there is moreenergy that can be released per unit time if oxygen is in excess to theavailable fuel. This results in higher average peak temperatures.Increasing the air flux increases the rate of forward convective heattransfer and that results in a faster front velocity (faster mass rateof destruction) at a given GAC concentration.

The GAC experiments demonstrate that:

-   -   1. GAC mixed with soil can achieve temperatures greater than        1000° C.;    -   2. Moderate temperatures (260° C.) can be used to initiate the        reaction (low energy requirements); and    -   3. Duration of treatment is relatively short based on achievable        front velocity.

Example 2: Assessment of PFAS Destruction Methodology

Test results from Example 1 indicated GAC would be an ideal fuel to usebecause it would allow temperatures of over 900° C. to be achievedduring self-sustaining smoldering. The tests of Example 2 were performedto examine the fate of added PFAS during smoldering. Example 2 comprisestwo tests using PFAS contaminated GAC and two using PFAS contaminatedsoil. The PFAS compounds used were perfluorooctanoic acid (PFOA),perfluorooctanesulfonic acid (PFOS), perfluorohexanesulfonic acid(PFHxS), perfluorononanoic acid (PFNA), perfluorobutanesulfonic acid(PFBS), and perfluoroheptanoic acid (PFHpA).

Sample Preparation

As shown in Table 2, stock solutions were prepared for tests 2-1 to 2-3that used three PFAS compounds (PFOA, PFOS, PFHxS), and test 2-4 usedsix PFAS compounds (PFOA, PFOS, PFHxS, PFNA, PFBS, and PFHpA).

TABLE 2 Amount of PFAS Compound Used for Stock Solutions Test NumberPFOA (g) PFOS (g) PFHxS (g) PFNA (g) PFBS (g) PFHpA (g) 2-1 0.64600.0988 0.2660 — — — 2-2 0.6460 0.0988 0.2660 — — — 2-3 10.2 1.56 4.2 — —— 2-4 5.1 0.78 2.1 5.1 0.516 5.1

For tests 2-1 and 2-2, GAC was added to each of the stock solutionbottles. The bottles were placed on a shaker table for four days at 170rpm. Each bottle was drained under vacuum through a 12.5 cm, coarsefilter paper (09-790-12E, Fisher Scientific). The GAC was then placed ina covered polypropylene container. This was completed for each of the 12bottles.

For tests 2-3 to 2-4, topsoil was dried in the oven at 105° C. for 24hours, crushed using a mortar and pestle, and then sieved using astandard set of sieves and the mechanical sieve shaker. Grains largerthan sieve #10 were removed from the soil. Remaining soil was thenblended to create a homogenized mixture.

Dried topsoil was added to the stock solution. The carboy was shakenperiodically over four days to re-suspend the soil and maximize thesorption of PFAS to the soil. Silicone tubing and a peristaltic pumpwere used to remove the soil and stock solution from the carboy. Ifdrained soil still contained significant free water, it was placed as athin layer in a tray until the moisture content decreased to less than20%. After this, the lid was placed on the container to keep residualwater in the soil. Moisture content was determined following the ASTMStandard D2974-14, Standard Test Methods for Moisture, Ash, and OrganicMatter of Peat and Other Organic Soils.

Smoldering Experiments

The experimental procedure for the smoldering column tests was the sameas described in Example 1. Table 3 provides the experimental parametersfor the four tests performed.

TABLE 3 Fuel, Fuel Ratio and Air Flux Used for Assessment of PFASDestruction Ratio (g fuel/ Air Test Porous kg sand Flux Number FuelMedia or soil) (cm/s) 2-1 GAC Sand 43 5.0 (with 3 PFAS) 2-2 GAC Sand 395.0 (with 3 PFAS) 2-3 GAC Soil 48 5.0 (with 3 PFAS) 2-4 GAC Soil 48 5.0(with 6 PFAS)

For tests 2-1 and 2-2, the same procedure was followed to prepare thesmoldering fuel and sand mixture as described in Example 1. For tests2-3 to 2-4 the same procedure was used; however, the coarse silica sandwas replaced with a mixture of 28% topsoil, 47% medium sand, and 25%coarse sand. For these measurements, the dry weight of the topsoil wasused to create a soil mixture which would represent common field soils.

The same procedure was used to conduct smoldering tests as described inExample 1. However, for these tests, additional emission collectionsystems were setup to monitor for PFAS emissions.

Results and Discussion Sand Treatment

The objective of test 2-1 and 2-2 was to assess if three target PFAS(PFOA, PFOS and PFHxS), could be destroyed when adsorbed onto GAC thatis then mixed with sand. Stock solutions of these three compounds weremade for each test and the amount of PFAS absorbed from each stocksolution by the GAC is presented in Table 4.

TABLE 4 PFAS Concentration in Stock Solution Before and After GACAddition Test 2-1 Test 2-2 PFAS Before After Before After Compound GACGAC GAC GAC PFOA 530 <0.2 551 0.083 PFOS 127 <0.2 133 0.046 PFHxS 215<0.2 241 0.025 All values in mg/L

The results indicate that GAC removed >99% of the PFAS in the stocksolutions. The difference in PFAS concentration in the stock solutionbefore and after GAC addition was used to calculate the total massabsorbed to the GAC and in the columns based on the amount of GAC used.

Table 5 presents the smoldering summary for both tests. The GAC to sandratio was adjusted to reflect the moisture content of the GAC.

TABLE 5 Smoldering Summary for Test 2-1 & 2-2 Avg. Avg. Ratio MoisturePeak Front Avg. Test (g GAC/ Air Flux Content Temp Velocity O₂ Temp.Self- No. kg sand) (cm/s) (%) (° C.) (cm/min) (%) Trend Sustaining 2-143 5 14 908 0.69 5 Steady Yes 2-2 39 5 21 950 0.68 6 Steady Yes

Based on these results, the moisture content reduces the average peaktemperature slightly, but otherwise has little impact on front velocity.

Table 6 shows the pre- and post-analytical results and indicates that noPFAS compounds were present within detection limits (D.L.) in the sandand ash after treatment.

TABLE 6 Pre and Post PFAS Concentrations Test 2-1 Test 2-2 PFAS Pre PostPre Post PFOA 590,000 <0.4 510,000 <0.4 PFOS 140,000 <0.4 120,000 <0.4PFHxS 240,000 <0.4 220,000 <0.4 All Results in ug/kg D.L. 0.4 ug/kg

Soil Treatment

Surrogate soil comprising a mixture of topsoil, medium silica sand(20-30 mesh) and coarse silica sand (16 mesh) was used in tests 2-3 and2-4. Test 2-3 used a mixture of 30% topsoil, 45% medium sand and 25%coarse sand, whereas test 2-4 used a mixture of 23% topsoil, 47% mediumsand and 25% coarse sand. Test 2-3 used the same three PFAS used intests 2-1 and test 2-2, and test 2-4 included three additional PFAS:PFNA, PFBS and PFHpA. The objective of these tests was to assess thetreatment of PFAS that was adsorbed to the soil, and the use of GAC tocreate smoldering conditions to support their destruction. In addition,these tests further evaluated gas emissions for breakdown by-productsand degree of PFAS destruction. The GAC ratio was targeted at 50 g/kgsoil to achieve temperatures greater than 1000° C. as observed in PhaseI tests.

Table 7 presents a summary of the smoldering data for tests 2-3 and 2-4and includes the moisture content and initial GAC composition. Theresults indicate that a self-sustaining reaction was obtained withtemperatures in excess of 1000° C. Tables 8 and 9 show the analyticaldata for tests 2-3 and 2-4, respectively, and demonstrate almostcomplete removal of all added PFAS.

TABLE 7 Smoldering Summary Avg. Avg. Peak Front Avg. GAC Air Flux M.C.Temp. Vel. O₂ Temp. Self- Test (g/kg) (cm/s) (%) (° C.) (cm/min) (%)Trend Sustaining 2-3 48 5 10.8 1016 0.63 5 Steady Yes 2-4 50 5 5.7 10640.72 6 Steady Yes

TABLE 8 Test 2-3 Soil Analytical Results PFAS (mg/kg) Sample PFHxS PFOAPFOS Blank Soil N.D. N.D. N.D. PFAS Loaded Soil 16.86 13.41 23.3 LoadedSoil with Sand & GAC  7.06  6.14  9.54 Post-Treatment Ash/Soil N.D.N.D.* N.D. Notes *2 of 3 samples were non-detect for all 3 PFAScompounds. 1 sample had a measured PFOA concentration of 0.0002 mg/kg.N.D. = not detected at Detection Limit of 0.00005 mg/kg

TABLE 9 Test 2-4 Soil Analytical Results PFAS (mg/kg) Sample PFBS PFHpAPFHxS PFOA PFNA PFOS Blank Soil N.D. N.D. N.D. N.D. N.D. N.D. PFASLoaded Soil 3.19 13.32 10.84 14.91 28.73 10.87 Loaded Soil with Soil &1.3   9.75  7.21 11.49 25.58  6.67 GAC Post-Treatment Ash/Soil N.D. N.D.N.D. N.D. N.D. N.D. N.D. = not detected at Detection Limit of 0.0005mg/kg

Based on Examples 1 and 2 it may be concluded that:

-   -   1. GAC can be used to support smoldering combustion to achieve        temperatures that destroy PFAS when added to soils at 40 to 60        g/kg.    -   2. PFAS absorbed to GAC or soils can be treated via smoldering        combustion resulting in non-detectable levels in soils, sand and        ash.    -   3. GAC smoldering can be initiated with low heat (260° C.) over        short time periods, and the smoldering front velocity is        sufficiently fast to be practicable at larger scales.

What is claimed is:
 1. A method for remediating contaminated soil andgroundwater, the method comprising: selecting a treatment material;creating a smolderable mixture of contaminant, treatment material andsoils; allowing the smolderable mixture to absorb and concentrate thecontaminant; heating a portion of the smolderable mixture; forcingoxidant through the smolderable mixture; initiating a self-sustainingsmoldering combustion of the smolderable mixture to destroy or removethe contaminant; and terminating the source of heat applied to thesmolderable mixture.
 2. A method according to claim 1, furthercomprising propagating the combustion away from the point of ignition ofthe combustion.
 3. A method according to claim 1, wherein thecontaminant is selected from a group consisting of perfluoroalkylsubstances, polyfluoroalkyl substances, dioxins, polychlorinatedbiphenyl compounds, pesticides, herbicides, volatile organic compounds(VOCs), and semi-volatile organic compounds (SVOCs), and combinationsthereof.
 4. A method according to claim 1, wherein the treatmentmaterial is selected from the group consisting of vegetable oil,hydrocarbons, tar, activated carbon, coal, charcoal, polymers,surfactants, sand, soils, silt, loam, fill, cobbles, gravel, crushedstone, glass, ceramics, zeolite, woodchips and combinations thereof. 5.A method according to claim 1, wherein the soil contains thecontaminants, and wherein the soil containing the contaminants is mixedwith the treatment material to create a smolderable mixture.
 6. A methodaccording to claim 1, wherein the treatment material is admixed belowthe ground using a variety of methods selected from the group consistingof trenching, large diameter auger, excavation, caisson, injection,jetting, fracking, vibrating beam, tremmie, soil mixing and combinationsthereof.
 7. A method according to claim 1, wherein the smolderablemixture is combusted in place.
 8. A method according to claim 1, whereinthe smolderable mixture is removed and combusted above the ground.
 9. Amethod according to claim 1, wherein the smolderable mixture is createdand combusted above the ground.
 10. A method according claim 1, whereinforcing oxidant through the smolderable mixture includes injecting airinto the smolderable mixture through an injection port.
 11. A methodaccording to claim 1, wherein initiating self-sustaining smolderingcombustion includes applying heat to the smolderable mixture from atleast one internal conductive heating source in direct contact with thesmolderable mixture.
 12. A method according to claim 1, whereininitiating self-sustaining smoldering combustion includes applying heatto the smolderable mixture from at least one convective heating sourcecoupled to the smolderable mixture.
 13. A method according to claim 12,wherein at least one convective heating source is external to thesmolderable mixture.
 14. A method according to claim 12, wherein atleast one convective heating source is located within the smolderablemixture.
 15. A method according to claim 1, wherein initiatingsmoldering combustion includes applying radiative heat to thesmolderable mixture.
 16. A method according to claim 1, wherein forcingoxidant through the smolderable mixture includes injecting air into thesmolderable mixture through a plurality of injection ports.
 17. A methodaccording to claim 1, wherein forcing oxidant through the smolderablemixture includes creating a vacuum to suck oxidant through thesmolderable mixture.
 18. A method according to claim 1 furthercomprising performing the smoldering combustion at a temperature withina range between 200 and 2000 degrees Celsius.
 19. A method according toclaim 1, further comprising forcing oxidant through the smolderablemixture at a linear velocity of between 0.0001 and 100 centimetres persecond.
 20. A method according to claim 1, wherein the treatmentmaterial is a liquid.
 21. A method according to claim 1, wherein thetreatment material is a slurry.
 22. A method according to claim 1,wherein the treatment material is a solid.